(a) Field of the Invention
The present invention relates to a distributed feedback semiconductor laser device and a multi-wavelength laser array which can be utilized as a signal source for wavelength division multiplexing (WDM) telecommunication, more in detail to those easily and economically fabricated, and a method for fabricating the same.
(b) Description of the Related Art
A multi-wavelength laser device has been attracting public attention, which generates a plurality of laser rays having different wavelengths as a signal source for the WDM telecommunication for transmitting a plenty of information through a single optical fiber by using the wavelength division multiplex technique.
In a conventional distributed feedback semiconductor laser device (DFB-LD), (1) a process of changing a pitch (space period) of a diffraction grating and (2) a process of changing a thickness or a composition of an active layer have been proposed for changing or controlling the lasing wavelength based on the Bragg""s reflection equation as follows:
p=(N/xcex)/(2neq), 
wherein xe2x80x9cpxe2x80x9d is a pitch of the diffraction grating, xcex is a transmission wavelength, xe2x80x9cNxe2x80x9d is an order of the diffraction, and neq is an effective refractive index (or equivalent refractive index).
The process of changing the lasing wavelength by changing the pitch of the a diffraction grating utilizes a principle that only a ray having an wavelength satisfying the Bragg""s reflection equation is transmitted with the increased strength in the diffraction grating. In the process, the diffraction grating having a pitch for increasing the strength of a ray with a desired wavelength and transmitting the ray is mounted in a resonator. For example, the change of the pitch of the diffraction grating by 0.01 nm varies the lasing wavelength by about 0.06 nm.
For fabricating the diffraction grating with the desired pitch, a layer of the diffraction grating is conventionally patterned by exposure with an electron beam lithography or by exposure using a master mask obtained by using the electron beam lithography.
In the other process, the lasing wavelength is varied by changing the thickness or the composition of the active layer to modify the equivalent refractive index while keeping the pitches of the diffraction grating constant. The composition of the active layer is changed by using a selective area growth method utilizing an MOCVD (metal organic chemical vapor deposition) process.
However, in the first process, it is extremely difficult that the requirement of the strict pitch control of the diffraction grating is satisfied during the fabrication of the diffraction grating having a frequency spacing with 100 GHz (about 0.8 nm with 1.5 xcexcm wavelength range) level required for the high density WDM telecommunication system.
In order to conduct the strict pitch control, the expensive electron beam alignor is required, thereby dissatisfying the economical standard because the equipment cost is increased and the productivity is decreased.
Further, in the second process, the strict control of the thickness or the composition of the active layer by using the selective area growth method is difficult in reality, and the productivity thereof is low.
Accordingly, a new process in place of the conventional method of fabricating the distributed feedback semiconductor laser device has been demanded.
In view of the foregoing, an object of the present invention is to provide a distributed feedback semiconductor laser device which can generate different lasing wavelengths and a lasing structure easily fabricated, and a method for fabricating the same.
Thus, the present invention provides, in a first aspect thereof, a distributed feedback semiconductor laser device including an active layer, a diffraction grating disposed in a vicinity of the active layer and having a substantially uniform space period and a distributed feedback function, and a functional layer disposed in a vicinity of the diffraction grating and the active layer and having a function of controlling a refractive index of the active layer, whereby the functional layer controls a lasing wavelength of the active layer (hereinafter referred to as xe2x80x9cfirst inventionxe2x80x9d).
In accordance with the first invention, the layer having the function of controlling the equivalent refractive index can generate a plurality of the lasing wavelengths different among one another and easily controllable.
The present invention provides, in a second aspect thereof, a multi-wavelength laser array including a common substrate, and a plurality of ridge wave guide type semiconductor laser devices disposed in an array overlying the common substrate, wherein each of the semiconductor laser device includes the functional layer as defined in claim 1 (hereinafter referred to as xe2x80x9csecond inventionxe2x80x9d).
In accordance with the second invention, the distributed feedback semiconductor laser device of claim 1 disposed in the array overlying the common substrate can generate the lasing wavelengths different among one another to provide the multi-wavelength laser array which is easily fabricated and economical.
The present invention further provides, in third and fourth aspects, methods for fabricating the distributed feedback semiconductor laser device of the first invention and the multi-wavelength laser array of the second invention.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.